Thermal switch

ABSTRACT

A thermal switch of the type dependent on the differential longitudinal expansion of two separate elongated elements. Contact means to be operated are responsive to the angular orientation of a rocker member, and to that member the two elements are connected to determine that orientation. An appropriate means restrains the rocker member against movement in one direction transverse to the elements, and spring means maintain under tension the restraining means and both the elements. At least one element being subjected to efficient electrical heating, the switch may constitute a thermal relay; neither being so subjected, but the elements being of different thermal expansibilities, the switch may constitute a thermostat.

United States Patent 191 Broekhuysen 1 1 Feb. 13,1973

[22] Filed: Feb. 23, 1972 r 211 Appl. No.: 228,651

[52] US. Cl. ..337/l23, 337/124, 337/128 [51] Int. Cl. ..ll0lh 61/01 [58] Field of Search ..337/l23, 124, 125, 126, 127, 337/131,132, 133,139, 141, 382, 383, 386,

[56] 7 References Cited 9 2,700,084 1/l955 Broekhuysen ..337/1 24 2,948,788 8/1960 Broekhuysen ..337/124 Primary ExaminerHarold Broome Attorney-Charles T. Jacobs [57] ABSTRACT A thermal switch of the type dependent on the differential longitudinal expansion of two separate elongated elements. Contact means to be operated are responsive to the angular orientation of a rocker member, and to that member the two elements are connected to determine that orientation. An appropriate means restrains the rocker member against movement in one direction transverse to the elements, and spring means maintain under tension the restraining means and both the elements. At least one element being subjected to efficient electrical heating, the switch may constitute a thermal relay; neither being so subjected, but the elements being of different thermal enpansibilities, the switch may constitute a thermostat.

10 Claims, 18 Drawing Figures TI-IERMAL switch This invention relates to thermal switches of the type whose operation depends on the differential longitudinal expansion of two separate elongated elements, that differential expansion being subjected to mechanical multiplication and, as so multiplied, used to operate electrical contacts.

One general approach to a thermal switch of this type actually constituting a thermal relay, since one of its two elongated elements is electrically heated is.

shown in U.S. Patent No. 1,773,708 to Whittingham. Therein each of the two elements is flexible and under tension. A rocker member in the mechanical multiplication system, however, not only is pivotally connected to by each of the elements, but also is in turn connected to the general supporting structure through shafted pivots resulting in substantial friction in a system which, for good stability and accuracy, should be frictionless as well as totally free of play.

A different general approach to such a switch is shown in a number of patents to me, of which examples are U.S. Pat. Nos. 2,700,084, 2,809,253 and 2,948,788, each showing a thermal relay. In each of these the effective pivotings required by the multiplication system are carried out wholly' by the use of leafspring (or equivalent) arrangements, thereby providing the requisite freedom from friction and play. In each, however, one of the two elongated elements is under compressive stress; this entails the use, in at least some of the pivotings, of leaf springs (or equivalents) which, to provide assurance against buckling, are relatively short and stiff. Such springs tend to occasion not only unwanted mechanical impedance to the intended contact motion but also undesirable heat transfer between the two elements and/or between one element and other components. I

Each of the first twoof the abovementioned patents to me employsin its mechanicalmultiplication system a rocker member which, to avoid the abovementioned disadvantages, is free of connection (other than for means exerting restraint against unidirectional motion) with the general supporting structure, being instead carried by the elongated elements-or, when only one of them is rigid, by that one. For the preservation of stability and accuracy, however, this has entailed the connection of the rocker member to a rigid one of those elements through an effective pivot formed by two leaf springs in a special and not-inexpensive crossed-spring arrangement.

It is an object of this invention toprovidea thermal switch of the type under discussion which is at once frictionless and free of play, without requirement for leaf springs or equivalents of such proportions as to occasion substantial impedance or undesirable inter-component heat transfer, and yet simple and inexpensive.

It is an object of this invention to provide, for a thermal switch, a basic mechanical design applicable, with minor additions or substitutions of components, to a wide variety of uses. It is an object to provide such a design involving low switch-fabrication cost, calling for a minimum amount of skilled labor, and requiring relatively inexpensive tooling. It is an object to provide such a design which is nevertheless appropriate to the production of switches of high sensitivity and accuracy, inherent stability and great uniformity. Other and allied objects will appear from the following brief and detailed descriptions and from the appended claims.

In an important aspect the invention comprises, in a thermal switch having a supporting structure and including contact means to be operated, the combination of (A) a pair of elongated elements, at least one of which is characterized by appreciable thermal expansibility, connected at their first ends to the supporting structure and extending in generally similar directions, (8) a rocker member to the angular orientation of which the contact means are responsive, the elements being connected at their'second ends to the rocker member at respective relatively closely spaced locations thereon lying along a line transverse to the elements, (C) means, connected to the rocker member from a location on the supporting structure displaced from the rocker member in one direction along the above-mentioned transverse line, for restraining the rocker member against movement in the opposite direction while permitting change of its angular orientation, and (D) spring means, interposed between the rocker member and the supporting structure, for maintaining under tension both of the elements and the restraining means.

The force exerted by each element on the rocker member may be essentially confined to the tension under which that element is maintained. For each element there may be provided means in the regions of the ends of that element for so confining that force.

In a thermostat according to the invention one of the elements may be characterized by substantial, and the other by small, thermal expansibility. In a relay according to the invention there may be provided electrical means for heating at least one of the elements. In such a relay that heated element may be of substantially higher per-unit-length resistance to heat flow in its endward portions than in its intervening central portion. Other aspects of the invention will appear in the follow ing detailed description and the appended claims.

In that description reference is had to the accompanying drawings, in which:

FIG. 1 is a front elevational view of a relay embodying the invention;

FIGS. 2, 3 and 4 are cross-sectional views respective ly taken along the lines 2-2, 3-3 and 4-4 of FIG. 1;

FIGS. 5 and 8 are fractional views alternative to respective portions of FIG. 2;

FIG. 6 is an elevational view of a modified element substitutable in the structure of FIGS. 1-4;

FIG. 7 is a cross-sectional view taken along the line 7--7 of FIG. 6;

FIG. 9 is a schematic illustration of the relay of FIGS. 1-4; 7

FIG. 9a is a fractional illustration of a modification of the relay schematically illustrated in FIG. 9;

FIGS. 10 and 11 are force diagrams showing the forces in therelay of FIGS. l4 under two respective conditions;

FIGS. 12, l3, l4 and 15 are schematic illustrations of respective modifications of the relay illustrated in FIG. 9; and

FIGS. 16 and 17 are schematic illustrations of respective thermostats according to the invention.

FIGS. 1 through 4 illustrate an embodiment of my invention in a thermal relay which by way of example is shown as assembled on a base 1 of rigid insulating material. On top of the base near'its righthand end there may be a metallic bracket 8, held in place for example by staking and soldering of its horizontal leg to the upper end portion of a shouldered flat connector lug 8' which passes through the base from therebelow. In the vertical leg of the bracket 8 (that leg being disposed in a side-to-side plane as viewed in FIGS. 1 and2) there may be assembled an adjusting screw 8a, at the rear of which there iscarried the contact 9 which is adjustably positioned forwardly and rearwardly by screw 8 but which is operationally stationary. Behind the contact 9 is a contact 10, carried on the forward surfacerof a vertical lug 29 which hangs down from an arm 26 to be described below. The contacts 9 and 10, thermally operated by structure still to be described, form the thermal switch in this embodiment.

The base 1 forms part of a supporting structure of which furtherportions include a metallic bracket 2 held on top of the base 1 near its lefthand end, for example by staking and soldering of its horizontal leg to the upper end portion of a shouldered flat connector -lug 2' which passes through the base 1 from therebelow, the vertical leg of the bracket 2 being disposed in a fron tand-back plane as viewed in FIGS. 1 and 2. A still further portion of the supporting structure is a metallic anchor member 4 mounted on top of the base 1 a little to the left of the bracket 8; this anchor member, which may for example be formed from a single flat piece of metal, may have front and rear bracket portions comprising parallel side-to-side-extending vertical legs 5 and 6'respectively, whose lefthand portions are continuous with and are interconnected by a portion 7 V-shaped in plan and having a leftwardly extending apex. The anchor member 4 may include, vertically somewhat above its center, lugs 5' and 6'; the lug 5 may be folded. diagonally forwardly from the front vertical leg 5. for example to'be in plan-view alignment with the front half of the V-shaped portion 7, while the lug 6 may be folded diagonally rearwardly from the rear vertical leg 6 for example to be in plan-view alignment with the rear half of the portion 7.

A pair of elongated elements, 11 and 12, are connected at their righthand ends to the supporting structure, specifically to the anchor member 4, and each extends from that member leftwardly in a respective vertical plane. In this embodiment each of thosemembers is thermally expansible; the elements may comprise strips 11' and 12 of identical material, preferably metal. As shown in FIG. I, the front strip 11' throughout most of its length is not much wider than the lug 5', but it is substantially widened in each of its end portions. Each end portion is centrally provided with a rectangular aperture, llr at'the righthand end and 11s at the left-hand end; the element is connected at its right-hand end to the anchor member 4 by hooking of its aperture 1 1r over the lug 5', its extreme righthand portion then lying in at least substantial contact with the front surface of the vertical leg 5. At least elevationally therear strip 12 may be identical with the front strip 11'; the rear element will be connected at its righthand end to the anchor member 4 by hooking of its corresponding aperture over the lug 6, its extreme righthand portion then lying in at least substantial contact with the vertical rear surface of the vertical leg 6.

At their lefthand ends the elements 11 and 12 are connected to a vertical rocker member 20 (most fully seen in FIG. 3). Means hereinafter described maintain that rocker member in as leftward a position as is permitted by the elements 11 and 12, and in a predetermined front-and-back and up-and-down position but its angular orientation (as seen in FIG. 2) will be appreciably variable, and to that angular position the contact means 9-10 are responsive.

Thus that rocker member may form a vertical leg within an elongated and specially shaped arm 26, which may be folded rightwardly from the top edge of that member into a relatively long flat arm portion 27; the portion 27 may be tapered (see FIG. 2) so as to have a narrowed righthand end portion 28. From the rear edge of the portion 28 adjacent the righthand arm extremity there may be folded downwardly the lug 29 on the forward surface of which, as abovementioned, the contact 10 is carried. As illustrated in FIG. 2 the contact 10 is spaced slightly from the contact 9 i.e., the contact means 9-10 are in an open condition, which in this embodiment may be considered the normal condition. Were the angular orientation of the rocker member 20 and thus of the whole arm 26 (as seen in FIG. 2) to shift slightly clockwise, however, the contact means would be operated i.e., changed to a closed condition by forward movement of the contact 10. Thereafter were that orientation to shift counterclockwise back to such an orientation as illustrated, the contact means would be re-operated, this time to restore their normal open condition, by rearward movement of contact 10.

Against the lefthand surface of the rocker member 20 there may be held the rear portion of a vertically disposed leaf spring 30 (see especially FIG. 4); such holding may if desired be by spot-welding. Preferably, however, that leaf spring portion will simply be held in contact with that rocker-member surface by means necessarily provided for other purposes and hereinafter described; in that event a unique up-and-down and front-and-back relationship between leaf spring and rocker member may be established by providing, in the lefthand surface of the latter (see FIG. 3), small conical recesses 20 and, in the former, rightwardly extending conical bosses 30' (see FIG. 4).

The elements 11 and 12 are connected to the rocker member 20 at respective relatively closely spaced locations on the latter lying along a front-and-back line i.e., a line transverse to those elements. To provide for such connection thereto the rocker member 20 may be provided with front and back apertures 21 and 22 (see FIG. 3) through which there freely pass, respectively, the lefthand portions of those elements; those end portions pass on through apertures 31 and 32 (see FIG. 4) in the leaf spring 30, which are somewhat more restricted in front-and-back dimension than are the apertures 21 and 22. Immediately to the left of the portion of leaf spring 30 in which the apertures 31 and 32 are located there may be provided a retaining spring 34 which is of substantial width in the vertical direction, which in "plan isshaped like the Greek letter omega with its bulbous portion extending leftwardly, and which is biased toward widening (i.e., front-and-back enlargement) of that bulbous portion relative to its illustrated condition.

The retaining spring 34, except at the outer (i.e., front and back) portions 35 and 36 of its base, is substantially wider than the height of the apertures in the left-hand end portions of elements 11 and 12 (e.g., of aperture 11s); in those outer portions, however, it is sufficiently narrowed to enable the insertion of those portions in those apertures respectively the bulbous portion of the spring, for the purpose, being first compressed and then released. Upon completion of such release afterthe insertion the retaining spring 34 biases the lefthand end portion of element 11 forwardly against the front edge of the leaf-spring aperture 31, and the left-hand end portion of element 12 rearwardly against the rear edge of the leaf-spring aperture 32.

The rocker member may be spaced behind the bracket 2 above referred to, and may be disposed with its left-hand surface in substantial front-andback alignment with the left-hand surface of the vertical leg of that bracket. To that left-hand surface there may be secured, as by spot-welding, the forward portion of the vertical leaf spring 30 (see especially FIG. 4); a first result is that the up-and-down position of the rocker member is uniquely predetermined. Beyond that, the leaf spring 30 forms a means, connected to the rocker member from a location on the supporting structure displaced forwardly from that member along the frontand-back line above referred to, for restraining the rocker member from rearward movement along that line; at the same time, being flexible, that spring 30 leaves the rocker member free to shift appreciably leftwardly and rightwardly, and to rock i.e., to depart appreciably from the front-and-back orientation in which it appears in FIG. 2. That flexibility and thus those freedoms may be enhanced by providing in the spring 30, between bracket 2 and rocker member 20,- a substantial central aperture 30".

The supporting structure may include a vertical post 3, which is secured to the base 1 at apoint displaced leftwardly from the rocker member 20 and somewhat reardwardly from .the front-and-back center of that member, and which is provided with a groove 3' at approximately the elevation of the center lines of elements 1] and 12. The arm 26, in which the vertical rocker member 20 is comprised, may include to the left of that member a short horizontal portion 23 folded from the bottom of that member, an inclined portion 24 folded diagonally upwardly from the left-hand edge of portion 23, and a short horizontal portion 25 folded leftwardly from the portion 24 at about the elevation of the groove 3; the portions 24 and 25 may be narrowed relative to 23 and 20. The arm 26 may be provided at the 24-25 juncture with a hole 25' whose front-andback position is preferably on a line between the post 3 and the front-and-back center of the rocker member 20. Held under tension between the hole 25' and the groove 3' may be a coil spring 40. An effect of this spring40 is to bias leftwardly the rocker member 20, the rear portion of the leaf spring 30, and the retaining spring 34, and thus to place the elongated elements 11 and 12 under longitudinal v(in FIGS. 1 and 2, left-andright) tension; another effect of the spring 40 (resulting from the appreciable rearward displacement of post 3 mentioned above) is to bias the rocker member 20 rearwardly, and thus to place the leaf spring 30 under longitudinal (i.e., front-and-back) tension.

In the structure of FIGS. 1 through 4 the element 11 includes electrical heating means whose energization by electric current will serve to cause the heating and thus the longitudinal expansion of that element. Such means may be a winding 13 of suitable resistance wire, encircling but insulated from the strip 11 throughout a substantial intermediate portion of that strips length. To provide such insulation there may be placed against the front and back surfaces of that portion of the strip 11' thin mica cards 19, of greater width (i.e., up-anddown dimension) than that strip portion, that portion thus being sandwiched between those cards. The end portions of the cards may be narrowed and notched, and bands 17 wrapped therearound may serve to hold the cards in place; near each end portion a respective conductive band 15 may be wrapped around the cards and used as a respective terminal for the winding 13, which may be wound around the cards from one band 15 to the other.

The description of the FIG. 1-4 structure may be completed by noting that the lugs 8' and 2' may form terminals for the contacts 9 and 10 respectively the connection of the contact 10 to the lug 2' being through the arm 26, spring 30 and bracket 2, each of which is of course conductive. It may also be noted that the strips 11' and 12' are desirably of stainless steel; although their dimensions may be widely varied for purposes hereinafter referred to, those particularly illustrated in these figures may be taken as typically having a width (except in their end portions) of the order of 0.100 inch and a thickness of the order of 0.002 inch. In FIGS. 1 through 4 the thicknesses of the strips 11' and 12, as well as of the other portions of the elements 11 and 12, have been exaggerated for the sake of clarity of illustration.

Operationwise, it will first be readily understood tha so long as strips 1 1 and 12' are of the same material, or are otherwise caused to have the same temperature coefficientof linear expansion, the angular orientation of the rocker member 20 and thus the front-and-back position of the movable contact 10 will be independent of ambient temperature. (As ambient temperature rises the rocker member 20 and thus the whole arm 26 and the contact 10 will undergo leftward displacement, and vice versa but of much too small a magnitude to hurtfully de-align the contacts 9 and 10.) The illustration in FIG. 2 thus involves no significant assumption as to what the ambient temperature is but it does assume that each of the elements 1 l and 12 is at that temperature.

Upon the supply of current to the winding 13 the element 11 will be efficiently heated and will undergo appreciable longitudinal expansion; by radiation and other thermal couplings there will be a slight attendant heating and longitudinal expansion of element 12, but that will be relatively minute in other words, the differential expansion of the elements will be appreciable. This differential expansion will permit the appreciable leftward displacement of the rocker member 20 where element 11 passes through it, but will permit at most a minute leftward displacement of the rocker member where element 12 passes through it. The thus-permitted movements will of course take place, under the influence of the spring 40 and their effect will be to cause a clockwise shift of the angular orientations of the rocker arm 20 and of the arm 26, with the result that contact 10 will be moved forwardly into closure against the contact 9. In this contact movement advantage is of course taken of the many-times step-up ratio between the element separation at the rocker member on the one hand, and the length of the arm portion 27 on the other.

As in the case of any thermal relay of the longitudinal-expansion type, the time elapsing between the inception of current supply to the heater winding and the operation, of the contact means a parameter whose provision is usually the prime purpose of the relay is a function among other things of the thermal capacity of the heated element. In the FIG. 1-4 structure as above described that capacity is fairly small, with the result that the time mentioned above may be of the very general order of several seconds only. Increase of this, typically to the very general order of 20 seconds, may be achieved by thickening of the strip 11; preferably it will be achieved by placing against a surface or the surfaces of the strip 11', throughout little more than the longitudinal extent of the mica cards 19, a strip or respective strips .11" of material similar to 11, as fractionally shown in FIG. 5, without thickening Further increase of the time mentioned above may be achieved by resort, for the electrically heated element, to the structure shown in FIGS. 6 and 7 in which such an element is shown under the designation 111. Herein there again appears the strip 11'; throughout its intermediate longitudinal portion, however, it is enclosed within a casing 121 to whose inner surface, prior to final formation of the casing from an initially flat piece of metal, the strip 11 may be spot-welded. Forwardly of the strip 11' within the casing there may be, successively, a first plain mica card 119, a winding-carrying mica card 117, anda second plain mica card 119; except at its end portions the card 117 may be somewhat narrower than. the cards 119 and in those end portions it may be provided with respective terminal strips 115 from one to the other of which there extends, wound around 117, a heater winding 113.

After assembly of the cards the casing may be'formed by folding so as substantially to envelop them.

Still further increase of the delay time mentioned above maybe achieved by including, within the casing 121 of FIGS. 67, additional mass in the form of metal strips as shown and described in each of US. Pats. Nos. 2,700,084 and 2,948,788 above referred to.

In each of the structures of FIG. and of FIGS. 67 the heated element has endward portions (i.e., those just short of the widened end portions proper) of very small cross-section, contrasting with the relatively large aggregate metallic cross-section of the intervening central portion which is provided with the additional metallic strip or strips (FIG. 5), or at least with the metallic casing 12] (FIGS. 67). In other terms, those endward portions are of substantially higher per-unitlength resistance to heat flow than is the intervening central portion. Thus, quite without regard to' how good or poor may be the heat conduction from the end portions of the element to other parts of the structure,

the element itself provides a substantial minimization of the conduction of heat away from its heated portion.

perature rise of most of element 11 and minimizes the counteracting temperature rise of element 12.

In the structures above described the elements 11 and 12 are characterized by substantial flexibility the unheated element throughout its length, and the heated element at least in the endward portions referred to in the preceding paragraph. As a result, and quite without regard to the nature of the connections of the element ends to the supporting structure and to the rocker member, the forces which they exert on the rocker member are essentially confined, in the case of each element, to the longitudinal tension under which that element is maintained. This makes an important contribution to accuracy, stability and uniformity of operation.

While such flexibility of the elements at least in their endward portions insures the result just referred to, the structure according to the invention permits that result to be achieved even though in one or both of the elements one or both of the endward portions (as well as other portions) be essentially rigid provided the connection of the respective element end which is adjacent each rigid endward portion, to the supporting structure or the rocker member as the case may be, be an effectively pivotal connection. This is inherently true of the connections of the elements lefthand ends to the rocker member 20 in the FIG. 14 structure; it can be made true as to the connections of their righthand ends to the supporting structure by modifying the shape of the anchor member 4, for example to that shown in FIG. 8, wherein the numeral designations of that member and of each of its portions have been increased by 100. Therein the vertical legs and 106 are reoriented so that each converges, as it proceeds rightwardly, toward the other; the lugs 105' and 106, however, may still be in substantial alignments with the respective sides of the V-shaped portion 107.

Generically, there are provided, as to each element, means essentially confining the force exerted by that element on the rocker member to the tensionunder which that element is maintained thosemeans being in the regions of the ends of that element, and at each end consisting of a flexible endward portion in the element, or a pivotal connection of the element at that end, or both.

The forces exerted in the structure of FIGS. 1-4 may be readily analyzed, convenientlywith reference to the simplified schematic showing of that structure which appears as FIG. 9. Those forces prior to the closure of the contacts 9-10 are depicted in the force diagram of FIG. 10.

In FIG. 10 the forces exerted on the rocker member 20 by the elements 11 and 12 (forces which have been shown above to be essentially confined to forces longitudinal of those elements) are shown as F, and F, respectively; the force exerted by leaf spring 30 on the rocker member (limited, in view of the high flexibility of that spring, essentially to a force longitudinal thereof) is shown as F and the force exerted by the coil spring 40 on the rocker member is shown as F By reason of the alignment of spring 40 above described, force F, is exerted on the center of the rocker member. The resultant of F l and F at that center is a force F equal to F,+F, or to 2F F is of course opposed by an equal and opposite force F and F,,' is readily analyzed as necessarily being the resultant of F, and F Although with heating and expansion of the element 11 the angular orientation of the rocker member 20 will change, the magnitude of that change required for closure of the contacts 9-10 is so small that the force diagram of FIG. remains for all practical purposes valid until contact 10 comes to rest against contact 9. Then with only slight further heating and expansion of the element 11, F will decrease and F and F will increase until F has dropped to zero at which time the forces will have become those depicted in FIG. 11.

In FIG. 11 the dash-dot line L represents the center line of the contacts 9-10. F represents the force exerted on the rocker member where that member is intersected by the element 12 (i.e., where force F was exerted in FIG. 10); it is equal to 2P The point at which the projection of F intersects the line L (which is independent of the physical location of the point of contact engagement) is designated as P; for purposes of further analysis F is replaced by F,", of length similar to and aligned with F but proceeding rightwardly from P. There will be a force F directed rearwardly from P along line L, representing the pressure exerted by contact 9 against contact 10 and thus arm 26; the resultant of F and F which will lie along a line passing through P and the center of the rocker member, is shown as F and by it and F the magnitude of F is of course determined.

The force F although appearing in a rightwardly displaced position, is of course exerted on the center of the rocker member 20; it is necessarily opposed by an equal and opposite force F F may readily be analyzed as necessarily being the resultant of force F, (exerted by spring 40 as in FIG. 10) and that force, F which is now exerted by the spring 30. It may readily be shown that the absolute magnitude of F is equal to the sum of the absolute magnitudes of F and of F (FIG. 10).

After the force F has dropped to zero and the force conditions of FIG. 11 have accordingly been established, further heating and expansion of the element 11 will not change any of those conditions, including the magnitude of the contact-closing force F provided the connection of that element to the anchor member 4 or the rocker member 20 (or both) be such as to permit longitudinal shifting of the elements end portion relative thereto (as both those connections are in the structure of FIGS. 1-4) or, if neither is, provided that element be flexible enough to buckle sufficiently to accommodate the further expansion without generating an appreciable new force on the rocker member. (In most typical instances the magnitude of this shifting or buckling will at most be very small indeed.) This avoids the need for trappedspring" or other special constructions such as are frequently necessary in conventional structures to avoid effects otherwise entailed by excessive contactclosing forces. It may be noted that when the endconnection of the element is such as to permit any such longitudinal shifting as is mentioned above, it should also be such as the FIG. 1-4 structure is that in the reverse part of the cycle of operation (e.g., in the cooling of element 11) the components between which the shifting has occurred will inherently return to the precise interrelationship in which they were before the shifting occurred.

The foregoing detailed description and the figures to which it refers have by way of example disclosed normally open contacts which are to be delayedly closed in response to the supply of current to the relay. The basic structure is equally adaptable to the operation of normally closed contacts which are to be delayedly opened in response to such a supply of current or to the operation in the same relay of both normally open and normally closed contacts.

The latter is schematically illustrated in FIG. 9a, which is intended for optional substitution for the righthand portion of FIG. 9. In FIG. 9a the arm 26 carries on a rear surface, opposite contact 10 on front, a contact behind which a stationary contact 109 may be adjustably held in a bracket 108. It may be assumed that with the whole relay at ambient temperature the arm 26, in the absence of contact 109, would occupy the position shown in FIG. 9, wherein the contact 10, would be appreciably spaced from the contact 9, but that the contact 109 has been adjusted sufficiently forwardly not only to come into closure against contact 110 but also to move arm 26 and thereby to bring contact 10 almost into closure against contact 9 which forward adjustment of contact 109 will have been attended by a release of the tension on element 12. Upon the supply of current to the relay (i.e., to the winding on the front element 11) element 11 will of course expand (and the rocker member 20 will shift minutely leftwardly) and, just before element 11 has expanded sufficiently so that in the FIG-9 structure contacts 9-10 would be closed, the tension on element 12 will just have been restored; the next small increment of heating and element-11 expansion will result in the forward rocking of arm 26, with the opening of contacts 109-110 and the closure of contacts 9-10. I

To provide simply for an opening of normally closed contacts the FIG. 9-a structure may be used with con tacts 9 and 10 omitted. It may be noted, however, that in this structure during the time the tension on element 12 is released there will be subtracted, from the normal force F; longitudinal of spring 30, a force of essentially the same magnitude, F,,, as is added to F when the tension in element 11 is released after closure of contacts 9-10. While this is quite permissible it is nevertheless generally preferable, when the relays operating task is only to open normally closed contacts, to modify the FIG-9 structure by making the rear element 12 the heated one, rather than by modifying the positions of the contacts. This has been schematically illustrated in FIG. 12; therein the heating means H, which in FIG. 9 appears as part of the front element 11, appears instead as part of the rear element 12. With this structure the normally closed condition of the contacts 9-10 results, when the relay is at ambient temperature, in the front element 11 being relieved of tension and in the FIG.-l1 force diagram being applicable. Upon the supply of current to the relay (i.e., to the heating means H) element 12 will of course expand (and the rocker member 20 will shift minutely leftwardly) until tension on element 1] has just been restored; the next small increment of heating and element-11 expansion will result in rearward rocking of arm 26, with the opening of contacts 9-10 whereupon the FIG-10 force diagram will become applicable and will remain so while the rear element 12 remains heated.

It may be pointed out that the avoidance of reduction of F i.e., the assurance that any change in the longitudinal tension under which spring 30 is placed will be an increase is accomplished by having the stationary contact (e.g., 9) displaced from the arm-carried contact (e.g., 10) in the same direction (e.g., forwardly) as is the bracket2 to which the spring 30 is secured. Y

As has been heretofore and elsewhere pointed out with respect to relays employing two longitudinally ex- .pansible elements one of which is heated, it is sometimes objectionable for the two elements to be too a widely discrepant from each other in respect of thermal inertia. In any of the relays according to the present invention wherein the heated element is of large inertia, (as a result, for example, of the employment of such a structure as thatof FIG. 6-7) the other element may be provided with any suitable inertia-increasing means extending throughout its longitudinally intermediate portion. This has been schematically illustrated in FIG. 13; therein schematically appears the FlG.-9 relay, but with its heating means replaced by a heating means H stantial contact with the forward surface of the front element 1 1.

There are a variety of purposes for which it may be desirable to provide heating means for both of the elements; this has been illustrated in FIG. 15, wherein separate heating means H, and H are carried by the elements 11 and 12 respectively. By way of non-limitative example ofa purpose for which this may be done there may be mentioned a protective arrangement for a multi-phase electrical system, in which arrangement the two heating means H, and H (then preferably identical, and in identical elements 11 and 12) would be respectively connected with two of the phases so that as long as both of those phases were functioning normally the arm 26 would be maintained in a balanced or stationary position but in which a failure of either phase would cause the arm26 to rock forwardly or rearwardly, in either case to operate contactsfwhich would invoke a shut-down of the system. For such a purpose there might be provided additional contacts such as 109 and 110 already shown in FIG. 9a, but the adjustments would be such that both 9-10 and 109-110 would normally be open.

The basic structure of FlGS. l-4 is readily adaptable thermal expansibility. FIG 16 illustrates a thermostat with normally open contacts to be closed on temperature rise; in this case the element 11 may be a strip of I such material as is referred to hereinabove for relay use, while the element 12 may be a strip of alloy material characterized by the small expansibility. FIG. 17 on of large inertia, and with the unheated element 12 prothe other hand illustrates a thermostat with normally closed contacts to be opened on temperature rise; in that case the elements 11 and 12 will be reversed in natures from those of FIG. 16.

Thermal switches according to the invention are not confined to those specifically disclosed above, which are illustrative rather than limitative. Thus while FlGS. 5 and 6-7 have shown typical variations of the HO. l-4 relay structure for the purpose of increasing its delay time, it will be understood that, contrariwise, such time may be decreased by employing for the heated element a hot-wire element structure such as disclosed in U.S. Pat. No. 2,809,253 (and also in FIGS. 7-15 of U.S. Pat. No. 2,948,788) abovementioned.

Other modifications too will be suggested by my disclosure to those skilled in the art. Such modifications will not necessarily constitute departures from the spirit of the invention or from its scope, which I undertake to define in the following claims.

1 claim:

1. ln a thermal switch having a supporting structure and including contact means to be operated, the combination of (A) a pair of elongated elements, at least one of which is characterized by appreciable thermal expansibility, connected at their first ends to said supporting structure and extending in generally similar directions, (B) a rocker member to the angular orientation of which said contact means are responsive, said elements being connected at their second ends to said rocker member at respective relatively closely spaced locations on said member lying along a line transverse to said elements, (C) means, connected to said rocker member from a location on said supporting structure displaced from said rocker member in one direction along said transverse line, for restraining said rocker member against movement in the opposite direction while permitting change of its angular orientation, and (D) spring means, interposed between said rocker memberand said supporting structure, for maintaining under tension both of said elements and said restraining means.

2. The subject matter claimed in claim 1 wherein the tension under which each said element is maintained constitutes substantially the only force exerted by that element on said rocker member.

3. The subject matter claimed in claim 1 wherein the force exerted by each said element on said rocker member is essentially confined to the tension under which that element is maintained.

4. The subject matter claimed in claim 1 further including, for each element, means in the regions of the ends of that element for essentially confining the force exerted on the rocker member by that element to the tension under which that element is maintained.

5. The subject matter claimed in claim 1 wherein said restraining means comprises a flexible leaf spring.

6. A thermostat comprising the subject matter claimed in claim 1 wherein one of said elements is characterized by substantial, and the other by small, thermal expansibility.

7. A thermal relay comprising the subject matter claimed in claim 1 further including electrical means for heating one of said elements.

8. The subject matter claimed in claim 7 wherein said one element has endward portions of which at least one 

1. In a thermal switch having a supporting structure and including contact means to be operated, the combination of (A) a pair of elongated elements, at least one of which is characterized by appreciable thermal expansibility, connected at their first ends to said supporting structure and extending in generally similar directions, (B) a rocker member to the angular orientation of which said contact means are responsive, said elements being connected at their second ends to said rocker member at respective relatively closely spaced locations on said member lying along a line transverse to said elements, (C) means, connected to said rocker member from a location on said supporting structure displaced from said rocker member in one direction along said transverse line, for restraining said rocker member against movement in the opposite direction while permitting change of its angular orientation, and (D) spring means, interposed between said rocker member and said supporting structure, for maintaining under tension both of said elements and said restraining means.
 1. In a thermal switch having a supporting structure and including contact means to be operated, the combination of (A) a pair of elongated elements, at least one of which is characterized by appreciable thermal expansibility, connected at their first ends to said supporting structure and extending in generally similar directions, (B) a rocker member to the angular orientation of which said contact means are responsive, said elements being connected at their second ends to said rocker member at respective relatively closely spaced locations on said member lying along a line transverse to said elements, (C) means, connected to said rocker member from a location on said supporting structure displaced from said rocker member in one direction along said transverse line, for restraining said rocker member against movement in the opposite direction while permitting change of its angular orientation, and (D) spring means, interposed between said rocker member and said supporting structure, for maintaining under tension both of said elements and said restraining means.
 2. The subject matter claimed in claim 1 wherein the tension under which each said element is maintained constitutes substantially the only force exerted by that element on said rocker member.
 3. The subject matter claimed in claim 1 wherein the force exerted by each said element on said rocker member is essentially confined to the tension under which that element is maintained.
 4. The subject matter claimed in claim 1 further including, for each element, means in the regions of the ends of that element for essentially confining the force exerted on the rocker member by that element to the tension under which that element is maintained.
 5. The subject matter claimed in claim 1 wherein said restraining means comprises a flexible leaf spring.
 6. A thermostat comprising the subject matter claimed in claim 1 wherein one of said elements is characterized by substantial, and the other by small, thermal expansibility.
 7. A thermal relay comprising the subject matter claimed in claim 1 further including electrical means for heating one of said elements.
 8. The subject matter claimed in claim 7 wherein said one element has endward portions of which at least one is of substantially higher per-unit-length resistance to heat flow than is its intervening central portion.
 9. A thermal relay comprising the subject matter claimed in claim 1 further including electrical means for heating both of said elements. 